Multiply antibiotic-resistant strains of Clostridium perfringens were isolated from porcine feces. Strains that were resistant to tetracycline, erythromycin, clindamycin, and lincomycin were isolated, but no penicillin- or chloramphenicol-resistant strains were obtained. Typical minimal inhibitory concentrations for resistant strains were 16 to 64 mug of tetracycline per ml, 64 to >128 mug of erythromycin per ml, >/=128 mug of lincomycin per ml, and 16 to 128 mug of clindamycin per ml. Resistance to erythromycin was always associated with resistance to lincomycin and clindamycin. Minimal inhibitory concentrations were determined for 258 strains from six farms that used antibiotics in their feeds and 240 strains from five farms that did not use antibiotics. The results show that 77.9 and 22.7% of the strains from the former farms were resistant to tetracycline and erythromycin-clindamycin-lincomycin, respectively. The comparable data from the latter farms were 25.0 and 0.8%, respectively. Agarose gel electrophoresis failed to reveal a plasmid band that was common to the resistant strains but absent in the susceptible strains. Attempts to transfer tetracycline, erythromycin, and clindamycin resistance from one strain, CW459, were not successful. Antibiotic-susceptible mutants were not isolated from this strain, despite the use of a variety of curing agents.